Improve Spring Batch coverage in reference documentation

See gh-19211

Author: Jay Bryant

spring-boot-project/spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -2104,9 +2104,13 @@ The preceding example overrides the default factory, and it should be applied to
 
 [[howto-batch-applications]]
 == Batch Applications
-This section answers questions that arise from using Spring Batch with Spring Boot.
+A number of questions often arise when people use Spring Batch from within a Spring Boot application.
+This section addresses those questions.
 
-NOTE: By default, batch applications require a `DataSource` to store job details.
+[[howto-spring-batch-specifying-a-data-source]]
+=== Specifying a Batch Data Source
+
+By default, batch applications require a `DataSource` to store job details.
 Batch autowires a single `DataSource` in your context and uses that for processing.
 To have Batch use a `DataSource` other than the application’s main `DataSource`, declare a `DataSource` bean, annotating its `@Bean` method with `@BatchDataSource`.
 If you do so and want two data sources, remember to create another one and mark it as `@Primary`.
@@ -2115,25 +2119,53 @@ See {spring-batch-api}/core/configuration/annotation/EnableBatchProcessing.html[
 
 For more about Spring Batch, see the {spring-batch}[Spring Batch project page].
 
-
-
-[[howto-execute-spring-batch-jobs-on-startup]]
-=== Execute Spring Batch Jobs on Startup
+[[howto-running-spring-batch-jobs-on-startup]]
+=== Running Spring Batch Jobs on Startup
 Spring Batch auto-configuration is enabled by adding `@EnableBatchProcessing` (from Spring Batch) somewhere in your context.
 
-By default, it executes *all* `Jobs` in the application context on startup (see {spring-boot-autoconfigure-module-code}/batch/JobLauncherCommandLineRunner.java[JobLauncherCommandLineRunner] for details).
+By default, it executes *all* `Jobs` in the application context on startup (see {spring-boot-autoconfigure-module-code}/batch/JobLauncherCommandLineRunner.java[`JobLauncherCommandLineRunner`] for details).
 You can narrow down to a specific job or jobs by specifying `spring.batch.job.names` (which takes a comma-separated list of job name patterns).
 
-[TIP]
-.Specifying job parameters on the command line
-====
-Unlike command line option arguments that <<spring-boot-features.adoc#boot-features-external-config-command-line-args,set properties in the `Environment`>> (i.e. by starting with `--`, such as `--my-property=value`), job parameters have to be specified on the command line without dashes (e.g. `jobParam=value`).
-====
+See {spring-boot-autoconfigure-module-code}/batch/BatchAutoConfiguration.java[BatchAutoConfiguration] and https://github.com/spring-projects/spring-batch/blob/master/spring-batch-core/src/main/java/org/springframework/batch/core/configuration/annotation/EnableBatchProcessing.java[@EnableBatchProcessing] for more details.
 
-If the application context includes a `JobRegistry`, the jobs in `spring.batch.job.names` are looked up in the registry instead of being autowired from the context.
-This is a common pattern with more complex systems, where multiple jobs are defined in child contexts and registered centrally.
+[[howto-spring-batch-running-command-line]]
+=== Running from the Command Line
 
-See {spring-boot-autoconfigure-module-code}/batch/BatchAutoConfiguration.java[BatchAutoConfiguration] and https://github.com/spring-projects/spring-batch/blob/master/spring-batch-core/src/main/java/org/springframework/batch/core/configuration/annotation/EnableBatchProcessing.java[@EnableBatchProcessing] for more details.
+Running Spring Batch with Spring Boot from the command line differs from running Spring Batch by itself from the command line.
+Spring Boot uses a class called `JobLaunchingCommandLineRunner`.
+Spring Batch uses a class called `CommandLineJobRunner`.
+They work a bit differently. However, for the most part, the differences do not cause trouble.
+However, they parse command line arguments differently, which can cause trouble, as described in the next section.
+
+==== Passing Command-line Arguments
+
+Spring Boot uses `--` (two hyphens) to signal application arguments.
+Spring Batch uses a single hyphen as a special marker on the `jobParameters` argument.
+This section explains how to reconcile that difference when you use the `jobParameters` argument for Spring Batch within a Spring Boot application.
+
+If you run Spring Batch with Spring Boot, Spring Boot strips the first `-` character from each command line argument.
+For example, `--exampleArgument` becomes `-exampleArgument`.
+Whether a command-line option has one hyphen or two often makes no difference in Spring Boot.
+However, in Spring Batch, putting a single `-` character before the `jobParameters` parameter indicates that Spring Batch should not use the `jobParameters` value as the identifier for the `Job`.
+Best practice is to use the `jobParameters` value as the identifier for the `Job`, so this issue may cause problems.
+To avoid the issue, you should generally use no `-` characters for the command-line options that you pass to Spring Boot on behalf of Spring Batch, as shown in the following example:
+
+[source]
+someParameter="someValue"
+
+However, if you mean to not use the `someValue` value as the identifier for the `Job`, use two hyphens, as shown in the following example:
+
+[source]
+--jobParameters="someValue"
+
+In the second example, Spring Boot passes the parameter to Spring Batch as `-jobParameters="someValue"`, and `someValue` is used as a non-identifying job parameter.
+
+[[howto-spring-batch-storing-job-repository]]
+=== Storing the Job Repository
+
+Spring Batch requires a memory store for the `Job` repository.
+If you use Spring Boot, you must use an actual database. Note that it can be an in-memory database.
+See https://docs.spring.io/spring-batch/trunk/reference/html/configureJob.html#configuringJobRepository[Configuring a Job Repository].